When I have done training for Zend Framework, one of the things that mystifies students to some extent is the whole plugin architecture and where things can go. There has been several articles written about it, but they tend to use code to describe it. [...] I had found that when I drew out the request lifecycle that it helped the students understand it better.
His diagram (seen here) lays out the full execution relationship for the request structure including where plugins, action helpers and controllers fit in the mix. He also describes it in more detail, mentioning some of the variations that could happen along the way.